Stanford Synchrotron Radiation Lightsource

(a) Superlattice wave vector (±e, 0, 0)_o as a function of x for La_1-xSr_1+xMnO₄. The dashed line for x > 0.5 is e = 2(1 - x) = 2n_e. (b) Linear-scale contour map (10% contours) of the scattering intensity around (10, 6, 0)_o for x = 0.525.

H scans through the (8, 4, 0)_o and (9, 4, 0)_o reflections for x = 0.40, 0.45,and 0.50 (T = 100 K). The (8, 4, 0)_o peak intensities are normalized to 10⁶. Below x=0.50, the (9, 4, 0)_o superlattice peak intensity decreases considerably with decreasing x. For x = 0.40, the peak is noticeably broadened and its intensity is ~10⁴ weaker than for x = 0.50 reflecting mixed-phase behavior

The strong electron correlations in transition metal oxides give rise to such phenomena as high-temperature superconductivity in layered cuprates and to stripe-like order in layered cuprates and nickelates. In the case of the manganites, an additional strong electron-lattice interaction leads to a very rich phase diagram in which structural, magnetic, and transport properties are intimately related. Colossal magnetoresistance (CMR) has been observed in the perovskite and double-layer manganites, but not in the single-layer system La_1-xSr_1+xMnO₄ (Mn214). Nevertheless, there are signs that the physics of Mn214 is similar to that of the perovskites. Information about the low-temperature structural phases of Mn214 can be expected to provide valuable insight into the role of dimensionality on the properties of the manganites, and also to contribute to a deeper understanding of single-layer transition metal oxides in general.

Simon Larochelle and co-workers have grown single crystals at Stanford's new Laboratory for Advanced Materials and carried out x-ray scattering studies at SSRL Beamline 7-2 to establish the low-temperature structural phase diagram of Mn214 [1]. For x = 1/2, this study provides a more complete picture than previous neutron [2] and x-ray [3] scattering experiments. An investigation of the effects of varying the eg electron concentration (ne= 1 - x) in the MnO2 layers revealed three distinct regions: disordered (x < 0.4), mixed-phase (0.4 < x < 0.5), and charge-ordered (x >0.5).  Above x = 0.5, the ordering of eg electrons is found to result in a structural distortion whose modulation period only depends on ne. Even though Mn214 does not exhibit CMR, this trend resembles findings for La_1-xCa_xMnO₃, which is a CMR material. This behavior furthermore is reminiscent of the charge- and spin-density wave order tendencies in the hole-doped layered cuprates and nickelates. The results of this study thus provide valuable quantitative information for tests of theories for CMR materials and layered transition metal oxides.